1. Field of the Invention
The present invention relates to a charging system and a charging apparatus which charge a secondary battery, and a battery pack provided with a secondary battery.
2. Description of the Background Art
FIG. 8 is a graphical representation, showing a method of managing a charging voltage and a charging current according to a prior art. FIG. 8 shows graphs, indicating that a constant-current and constant-voltage (CCCV) charge is given to a lithium-ion battery. Fluctuations are shown in a terminal voltage α1 of a secondary battery, a charging current α2 supplied to the secondary battery and an SOC (or state of charge) α3.
First, in the case of such a CCCV charge (e.g., refer to Japanese Patent Laid-Open No. H6-78471 specification), the charge starts from a constant-current (CC) charge range. Within the constant-current (CC) charge range, a predetermined constant current I1 is supplied as the charging current, so that a constant-current (CC) charge can be conducted. The constant current I1 is set, for example, to a current value obtained by multiplying seventy percent of “1C” by a parallel-cell number PN. “1C” is a current level at which a nominal-capacity value NC can be discharged through a one-hour constant-current discharge.
Thereby, if the terminal voltage of a battery pack's charge terminal comes to an end voltage Vf determined in advance at 4.2 volts per cell (=a series-cell number SN×4.2V: in the case where three cells are in series, for example, 12.6V), then a transition is made to a constant-voltage (CV) charge range. Then, the charging current's value is reduced so that it will not exceed the end voltage Vf. If this charging-current value falls to a current value I2 set according to the temperature, then a decision is made that it is fully charged. Thereby, the charging current's supply comes to a halt. In other words, this indicates that when the charging-current value has fallen to the current value I2, SOC α3 is a hundred percent.
The current value I2 is a decision value for detecting a full charge. In order to bring the charge capacity to a full charge, desirably, it should ideally be zero amperes. In the constant-voltage charge, however, the closer the secondary battery comes to the full charge, the less the charging current becomes, thus making the charge slower. Hence, if the current value I2 is set to zero amperes, it takes a long time to increase a little charge capacity. Therefore, the current value I2 is suitably set by striking a balance between the capacity of a charge and the time taken for the charge. The current value I2 is set, for example, to a current value of approximately (0.1 A×parallel-cell number PN). In this case, 0.1 A is set, for example, as 1/20CA for a battery capacity C.
FIG. 9 is a graphical representation, showing fluctuations in a positive-electrode potential Pp and a negative-electrode potential Pm with respect to a lithium reference in the case where a lithium-ion battery is charged. Its horizontal axis indicates SOC and the vertical axis indicates the electric potential. As shown in FIG. 9, when the lithium-ion battery is charged, the SOC rises. Along with this, the positive-electrode potential Pp heightens and the negative-electrode potential Pm lowers. In this case, the lithium-ion battery's terminal voltage corresponds to the difference between the positive-electrode potential Pp and the negative-electrode potential Pm. In other words, it is given by Pp−Pm.
As the SOC increases, the negative-electrode potential Pm decreases and reaches zero volts. At this time, the difference between the positive-electrode potential Pp and the negative-electrode potential Pm is equivalent to the terminal voltage. Herein, the battery's capacity is designed so that the negative-electrode potential Pm becomes an electric potential (e.g., approximately 0.1 volts) higher than zero volts when the SOC comes to a hundred percent. This is conducted by taking into account the dispersion of the electric current or temperature, or the like, at the time of a charge, and further, the dispersion of the weight, or the like, at the time of manufacturing. Thereby, a margin is given so that the negative-electrode potential can be prevented from dropping to zero volts or below, even if such dispersion occurs at the same time.
In other words, the design includes measures against such dispersion, and thus, the fact that it is substantially zero volts means including a range until the negative-electrode potential becomes approximately 0.1 volts.
The lithium-ion battery's terminal voltage is affected by the dispersion of the charging-current value, the temperature and the composition of an active material for the positive electrode and negative electrode. Hence, the end voltage of charge may be set to substantially 4.2 volts if lithium cobaltate or lithium nickelate is mainly used as the positive-electrode active material. If lithium manganate, or a manganese-system positive-electrode active material obtained by substituting manganese for a part of a positive-electrode active material composed of a plurality of metallic elements, is mainly used as the positive-electrode active material, then the end voltage of charge may be set to substantially 4.2 volts or above.
In addition, if the negative-electrode potential Pm declines to a negative potential, lithium ions which have moved from the positive electrode to the negative electrode are deposited as metallic lithium on the negative electrode's surface. Then, the metallic lithium deposited on the negative-electrode surface turns to a tree-shaped dendrite crystal, or a so-called lithium dendrite. It grows toward the positive electrode and penetrates, for example, a separator made of a resin material such as polyethylene. Then, it short-circuits the negative electrode and the positive electrode. As a result, a short-circuit current passing through the lithium dendrite may melt the separator, enlarge the short circuit's part and destroy the battery.
Therefore, in order to prevent the negative-electrode potential Pm from falling to a negative potential, the end voltage Vf is set so that the terminal voltage per cell will not exceed 4.2 volts if lithium cobaltate is used as the positive-electrode active material. If a manganese-system positive-electrode active material is used as the positive-electrode active material, it is set so that the terminal voltage per cell will not exceed, for example, 4.3 volts. For example, if lithium cobaltate is used as the positive-electrode active material, the end voltage Vf is set to 4.2V×the series-cell number SN. If lithium manganate is used as the positive-electrode active material, the end voltage Vf is set to 4.3V×the series-cell number SN. Hence, when the secondary battery is charged, the charging voltage is not supposed to exceed the end voltage Vf set in this way.
Besides, if the battery is further charged after the negative-electrode potential Pm has become zero volts, metallic lithium will be deposited. Thus, it cannot be charged any more, so that the state of charge at the time when the negative-electrode potential Pm comes to zero volts is designed to be the full-charge state (SOC: 100%).
With respect to a secondary battery, there are needs for increasing its battery capacity and for restraining its degradation so that its life can be secured. In order to increase the battery capacity of a secondary battery, there can be a method of heightening the charging voltage and charging the secondary battery at a constant voltage. However, as described above, if the charging voltage becomes higher than the voltage (hereinafter, referred to as the reference voltage) between the negative electrode and the positive electrode in the full-charge state where the lithium-reference electric potential of the negative electrode is substantially zero volts, then a lithium dendrite may be formed to short-circuit the negative electrode and the positive electrode. This short-circuit current can melt the separator, enlarge the short circuit's part and destroy the battery. Hence, a disadvantage arises in that the charging voltage cannot be heightened beyond the reference voltage.
Herein, as described above, the design includes measures against the above described dispersion, and thus, the fact that it is substantially zero volts means including a range until the negative-electrode potential becomes approximately 0.1 volts.
Incidentally, as described above, even if a lithium dendrite is formed, a short-circuit current flows and heat is generated, then the short circuit's part can be prevented from becoming larger by forming a heat-resistant porous insulating film between the negative electrode and the positive electrode. A secondary battery having such a function is known (e.g., refer to Japanese Patent Laid-Open No. H7-220759 specification). In this secondary battery, even if a short-circuit current passes and heat is generated, the short circuit's part is not supposed to enlarge. Hence, it would be possible to increase the battery's charge capacity by heightening the charging voltage beyond the reference voltage.
However, even if such a secondary battery is used in which the heat generated by a short-circuit current derived from a lithium dendrite cannot enlarge the short circuit's part, then when a charging voltage beyond the reference voltage is applied, lithium may be deposited partly on the negative electrode. Then, a lithium dendrite can be formed and deteriorate the secondary battery. This would present a disadvantage in that the demand cannot be met that the secondary battery be restrained from being degraded so that its life can be secured.